Noise induced hearing loss can be caused by continuous exposure to loud sounds over an extended period of time. Any sound at or over 85 dB may cause noise induced hearing loss. More young people are developing noise induced hearing loss as the popularity of personal portable media and multimedia devices, such as mp3 players, cellular phones, smart phones, and portable game consoles, is growing. In particular, research indicates that people listening to audio signals through earphones or headphones with a personal portable media or multimedia device on a street or a noisy environment are in great danger of suffering from noise induced hearing loss.
Typical audio files contain a large dynamic range of sound levels. For example, the dynamic range for audio files on CDs is around 90 dB and the dynamic range for audio files on DVDs is around 120 dB. Listeners usually adjust the volume of a media or multimedia player device such that they can comfortably enjoy the lowest or average sound levels, thereby causing potential hearing risk during the loudest sound levels. The hearing risk is compounded with extended listening time, which is common with today's personal portable media players with memory or hard disks containing, literally, days of music.
In general, people adjust the volume of the music at their comfort level when they start to listen to music. Every listener's comfortable level is different. It could be loud or soft. After adjusting the volume, listeners will often keep listening at the same volume level no matter how loud the volume set-up because listeners' ears will adapt to the adjusted volume level quickly.
In addition, with the large dynamic range of most audio files, even a volume level that appears to be in a safe region can contain elements that extend into levels that can increase the risk of hearing loss.
Often in the search for safer volume levels, there is a trade-off between safety and overall listening experience.
Some commercial devices such as the Apple iPod have the capability to limit the instantaneous power output from the device. This is not a satisfactory solution primarily because of the inevitable clipping and distortion that occurs in the louder portions of the audio when the volume is increased to properly hear the softer portions. Also, the max dB sound level presented to the ear is a function of the headset and so cannot be set independently by the music file or playing device.
Another technique that has been in use for decades is to perform sound leveling between songs so that, for example, a CD can be burned from sources with inconsistent sound levels. More recently, audio players such as iTunes 7 have the capability to change the overall sound setting for each song using a number stored within the song data file. This reduces the dynamic range of a CD or play list but does not solve the problem of dynamic range variation within a song. Accordingly, research is ongoing for devices and methods that can reduce the risk of noise induced hearing loss.